A power supply using a general switching converter requires a rectifying circuit for rectifying AC input power into DC power. In addition, a smoothing capacitor having a large capacitance value is used in the rectifying circuit so as to reduce a burden on a switching element by compensating instantaneous power failure or reducing ripples of input power. Securing hold-up time is another major factor for determining the capacitance of the capacitor. As shown in FIG. 2, the hold-up time refers to an amount of time for which an output voltage maintains a prescribed voltage range after an input is cut off.
Thus, when a capacitor having large capacitance is used as an output capacitor of a power factor corrector (PFC) used in a vehicle, ripples can be reduced, thereby supplying stable power. However, the size of an onboard battery charger is limited due to its space, and accordingly, there is a limitation on increasing capacitance.
Therefore, a capacitor having capacitance which can satisfy the hold-up time is usually selected. However, in a conventional PFC, there is a problem in that a capacitor having a capacitance value greater than that required is selected for improved hold-up time and reduced ripples.
FIG. 1 is a circuit diagram illustrating a circuit configuration of a conventional onboard battery charger.
The conventional onboard battery charger may include an AC power source 10, a PFC circuit 20, a phase shift full bridge 30, an output terminal inductor 40, an output terminal capacitor 50, a battery 60, and the like.
In the conventional onboard battery charger, the phase shift full bridge 30 may include a switch circuit 31, a transformer 32 and an output terminal diode 33. The PFC circuit 20, which may be referred to as a power factor improving circuit, may be configured to include an inductor 21, a switching element 22, a diode 23 and an output capacitor 24.
As shown in FIG. 1, the output capacitor 24 of the conventional PFC circuit 20 is simply connected to an output terminal of the PFC circuit 20, and therefore, the capacitance value of the output capacitor 24 cannot be controlled by changing the structure of the PFC circuit 20. That is, the voltage applied to both ends of the output capacitor 24 cannot be controlled in an instantaneous power failure of the AC power source 10, and a capacitor having a large capacitance value should be selected as the output capacitor 24 in order to maintain long hold-up time.
However, the output capacitor 24 having a simply large capacitance value increases the size of the onboard battery charger and reduces the output density, i.e., power density calculated by dividing a power by a volume, of the onboard battery charger. Further, the output capacitor 24 may cause a problem in the configuration of a high-voltage charge system package.